A list of 26 Species "Concepts"

Here is a working list of species concepts presently in play. I quote "Concepts" above because, for philosophical reasons, I think there is only one concept - "species", and all the rest are conceptions, or definitions, of that concept. I have christened this the Synapormorphic Concept of Species in (Wilkins 2003). More under the fold:

A Summary of 26 species concepts

There are numerous species "concepts" (i.e., conceptions of "species") at the research and practical level in the scientific literature. (Mayden 1997) has listed 22 distinct species concepts along with synonyms, which provides a useful starting point for a review. I have added authors where I can locate them in addition to Mayden's references, and instead of his abbreviations I have tried to give the concepts names, such as biospecies for Biological Species, etc. (following George 1956), except where nothing natural suggests itself. There have also been several additional concepts since Mayden's review, which I have added the views of Pleijel and Wu, and several new revisions presented in Wheeler and Meier (2000). I also add some "partial" species concepts - the compilospecies concept and the nothospecies concept. I distinguish between two phylospecies concepts that go by various names, mostly the names of the authors presenting at the time (as in the Wheeler and Meier volume). To remedy this terminological inflation, I have christened them the Autapomorphic species concept and the Phylogenetic Taxon species concept.

See: Phylospecies Principal authors: Nelson and Platnick (1981); Rosen (1979) Specification: A geographically constrained group of individuals with some unique apomorphous characters, the unit of evolutionary significance (Rosen); simply the smallest detected samples of self-perpetuating organisms that have unique sets of characters (Nelson and Platnick); the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character traits (Wheeler and Platnick 2000).

Synonyms: Internodal species concept, Hennigian species concept, Hennigian convention Principal authors: Hennig (1966; 1950); Kornet (1993) Specifications: Set of organisms between speciation events or between speciation event and extinction (Ridley 1989), a segment of a phylogenetic lineage between nodes. Upon speciation the ancestral species is extinguished and two new species are named.

Synonyms: Phylospecies (in part), Internodal species (in part), cladospecies (in part) Principal authors: Kornet (1993) Specifications: All organisms belonging to an internodon and its descendents until any subsequent internodon. An internodon is defined as a set of organisms whose parent-child relations are not split (have the INT relation).

8. Ecospecies

Synonyms: Ecotypes Related concepts: Evolutionary species sensu Simpson, Ecological mosaics Principal authors: Simpson (1961); Sterelny (1999); Turesson (1922); Van Valen (1976) Specifications: A lineage (or closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range.

Synonyms: Biospecies (in part) and evolutionary species (in part) Principal authors: Waples (1991) Specifications: A population (or group of populations) that (1) is substantially reproductively isolated from other conspecific population units, and (2) represents an important component in the evolutionary legacy of the species.

Synonyms: none Related concepts: Genealogical concordance species, genetic species (in part), biospecies (in part), autapomorphic species (in part) Principal author: Wu (2001b; 2001a) Specifications: A species formed by the fixation of all isolating genetic traits in the common genome of the entire population.

Synonyms: Biospecies (in part), cladospecies (in part), phylospecies (in part), internodal species Principal authors: Hennig (1966; 1950); Meier and Willman (1997) Specifications: A tokogenetic community that arises when a stem species is dissolved into two new species and ends when it goes extinct or speciates.

16. Internodal species

Synonyms: Cladospecies and Hennigian species (in part), phylospecies Principal author: Kornet (1993) Specifications: Organisms are conspecific in virtue of their common membership of a part of a genealogical network between two permanent splitting events or a splitting event and extinction

17. Least Inclusive Taxonomic Unit (LITUs)

Synonyms: evolutionary group (in part), phylospecies Principal authors: Pleijel (Pleijel 1999; Pleijel and Rouse 2000) Specifications: A taxonomic group that is diagnosable in terms of its autapomorphies, but has no fixed rank or binomial.

18. Morphospecies

Synonyms: Classical species, Linnaean species. Related concepts: Linnean species, binoms, phenospecies, monothetic species, monotypes, types, Taxonomic species Principal authors: Aristotle and Linnaeus, and too many others to name, but including Owen, Agassiz, and recently, Cronquist (1978) Specifications: Species are the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means (Cronquist). Contrary to the received view, this was never anything more than a diagnostic account of species.

Synonyms: hybrid species, reticulate species Related concepts: Compilospecies, horizontal or lateral genetic transfer Principal author: Wagner (1983) Specifications: Species formed from the hybridization of two distinct parental species, often by polyploidy.

Phylospecies

Synonyms: Autapomorphic phylospecies, monophyletic phylospecies, minimal monophyletic units, monophyletic species, lineages Related concepts: Similar to internodal species cladospecies, composite species, least inclusive taxonomic units. Principal authors: Cracraft (1983); Eldredge and Cracraft (1980); Nelson and Platnick (1981); Rosen (1979) Specifications: The smallest unit appropriate for phylogenetic analysis, the smallest biological entities that are diagnosable and monophyletic, unit product of natural selection and descent. A geographically constrained group with one or more unique apomorphies (autapomorphies). There are two versions of this and they are not identical. One derives from Rosen and is what I call the Autapomorphic species concept. It is primarily a concept of diagnosis and tends to be favoured by the tradition known as pattern cladism. The other is what I call the Phylogenetic Taxon species concept, and tends to be favoured by process cladists.

21. Phylogenetic Taxon species

See: Phylospecies Principal authors: Cracraft (1983); Eldredge and Cracraft (1980); Nixon and Wheeler(1990) Specifications: A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft); the least inclusive taxon recognized in a classification, into which organisms are grouped because of evidence of monophyly (usually, but not restricted to, the presence of synapomorphies), that is ranked as a species because it is the smallest 'important' lineage deemed worthy of formal recognition, where 'important' refers to the action of those processes that are dominant in producing and maintaining lineages in a particular case (Mishler and Brandon 1987).

22. Phenospecies

Synonyms: Phena (sing. phenon) (Smith 1994), operational taxonomic unit (OTU) Related concepts: Biospecies, genetic concordance species, morphospecies, non-dimensional species, phylospecies (in part), phenospecies, successional species, taxonomic species, quasispecies, viral species, genomospecies (bacteria) Principal authors: Beckner (1959); Sokal and Sneath (1963) Specifications: A cluster of characters that statistically covary, a family resemblance concept in which possession of most characters is required for inclusion in a species, but not all. A class of organisms that share most of a set of characters.

23. Recognition species

Synonyms: Specific mate recognition system (SMRS) Related concepts: Biospecies Principal author: Paterson (1985) Specifications: A species is that most inclusive population of individual, biparental organisms which share a common fertilization system

24. Reproductive competition species

Synonyms: Hypermodern species concept , Biospecies (in part) Principal author: Ghiselin (1974) Specifications: The most extensive units in the natural economy such that reproductive competition occurs among their parts.

Synonyms: Cynical species concept (Kitcher 1984) Related concepts: Agamospecies, genealogical concordance species, morphospecies, phenospecies, phylospecies Principal author: Blackwelder (1967), but see Regan (1926) and Strickland et al. (1843) Specifications: Specimens considered by a taxonomist to be members of a kind on the evidence or on the assumption they are as alike as their offspring of hereditary relatives within a few generations. Whatever a competent taxonomist chooses to call a species.

Mallet, J (1995), "The species definition for the modern synthesis", Trends in Ecology and Evolution 10 (7):294-299.

Mayden, R. L. (1997), "A hierarchy of species concepts: the denoument in the saga of the species problem", in M. F. Claridge, H. A. Dawah and M. R. Wilson (eds.), Species: The units of diversity, London: Chapman and Hall, 381-423.

Mayr, Ernst (1942), Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press.

------ (1963), Animal species and evolution. Cambridge MA: The Belknap Press of Harvard University Press.

Pleijel, Frederik, and G. W. Rouse (2000), "Least-inclusive taxonomic unit: a new taxonomic concept for biology", Proceedings of the Royal Society of London - Series B: Biological Sciences 267 (1443):627-630.

Regan, C. Tate (1926), "Organic evolution", Report of the British Association for the Advancement of Science, 1925:75-86.

Ridley, M (1989), "The cladistic solution to the species problem", Biology and Philosophy 4:1-16.

Rosen, Donn E. (1979), "Fishes from the uplands and intermontane basins of Guatemala: revisionary studies and comparative biogeography", Bulletin of the American Museum of Natural History 162:267-376.

Simpson, George Gaylord (1943), "Criteria for genera, species and subspecies in zoology and paleontology", Annals New York Academy of Science 44:145-178.

Strickland, Hugh. E., John Phillips, John Richardson, Richard Owen, Leonard Jenyns, William J. Broderip, John S. Henslow, William E. Shuckard, George R. Waterhouse, William Yarrell, Charles R. Darwin, and John O. Westwood (1843), "Report of a committee appointed "to consider of the rules by which the nomenclature of zoology may be established on a uniform and permanent basis"", Report of the British Association for the Advancement of Science for 1842:105-121.

Templeton, Alan R. (1989), "The meaning of species and speciation: A genetic perspective", in D Otte and JA Endler (eds.), Speciation and its consequences, Sunderland, MA: Sinauer, 3-27.

Turesson, Göte (1922), "The species and variety as ecological units", Hereditas 3:10-113.

Wagner, Warren H. (1983), "Reticulistics: The recognition of hybrids and their role in cladistics and classification", in N. I. Platnick and V. A. Funk (eds.), Advances in cladistics, New York: Columbia Univ. Press, 63-79.

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John, my head hurts now. Is there a book in here somewhere? If you haven't thought about it, perhaps you should consider converting this into a book. I'm afraid a fellow like me needs more exposition and context. I guess for now I'll just have to muddle through your references to improve my understanding. Ruefully....Scott

Wow. I knew this whole species concept thing was difficult, but I had no idea just how messy it really is. So respect for being able to put some sense into it! Anyway, sounds like a mighty interesting book you've got there...

Hi - I'm the Crovello that Fusilier mentions in his Oct 2 comment; I wasn't a student of Sokal but spent a postdoc year and a half with him at Kansas (66-'67) after my Ph.D. from Berkeley. Two references to include in the list are "The Units of Evolution: Essays on the Nature of Species" - 1991 MIT Press Edited by Marc Ereshefsky (see web page athttp://mitpress.mit.edu/catalog/item/default.aspttype=2&tid=8930
The second one is in the above collection and is the Sokal & Crovello paper in the American Naturalist where we attempt to develop an actual operational procedure to delimit a biospecies. (perhaps it still is kind of a unique paper in this regard; but I have not kept up with the literature and I went on to other things, e.g., quantitative biogeography, and introduced "OGU" (Operational Geographic Unit; of course!) Hope these references help you in filling out your concepts tree (with a few spider webs in it as some of the concepts themselves don't fit nicely into a hierarchy as others have commented in the blog).
NOTE TO FUSILIER:Please email me at TedCrovello@sbcglobal.net to tell me where we interacted back then. Best wishes to all. Ted

Wow.. This is really interesting.. Too bad I'm not teaching Evolution and Biodiversity anymore after this but, I'll make sure to read this page again, and again.. I hope the book will come out soon..
Thank you.

What's the difference between the "cladospecies", the "composite species", the "Hennigian species", and the "internodal species"? In all four, "species" is simply used as a synonym for "internode", and "speciation" for "cladogenesis", right?

The proponents of the LITU concept refuse to call LITUs "species". They treat "species" like all other ranks as an encumbrance that should be abolished. It could be called an anti-species concept. :-)

I have never seen "OTU" used as a synonym of any species concept. OTU is what the taxa entered in a data matrix of a phenetic or phylogenetic analysis are called, no matter how large they are. You'll find complaints in cladistic literature that people shouldn't use "suprageneric OTUs".

These various phylospecies conceptions overlap and some are just restatements of others (internodals, for example, are just Hennigian species). But they aren't equivalent. Cladospecies are nodes, not edges of a cladogram.

LITUs and rankless taxa like OTUs are what I call "species replacement conceptions". Like "deme" was once a rankless term, these things end up being very like species if not the traditional understandings exactly.

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Thanks for posting this compilation. This is a useful entrance into the literature.

I think you need to draw a clearer distinction between two versions of "the phylogenetic species concept": the autapomorphic or monophyletic species concept of Rosen (1979) (see also Donoghue. 1985. The Bryologist. 88:172-181) and what is now usually called the "the diagnosable species concept" of Nixon and Wheeler (1990) (see also Davis and Nixon. 1992. Systematic Biology. 41: 421-435). There is a fundamental disagreement between these two "phylogenetic species concepts" over whether cladistic analysis of individual organisms or alleles of those organisms has any role in discovering species. Specifically, Nixon & Wheeler and Davis & Nixon emphasize that relationships among individuals within a sexual population are reticulate, not hierarchic. It is therefore not meaningful to speak of "a monophyletic group of organisms" or a "monophyletic species" composed of sexually-reproducing individuals since monophyly can only describe relationships in a hierarchic system. Thus Nixon & Wheeler would strongly object to your characterization of their species concept as ". . . the least inclusive taxon recognized in a classification, into which organisms are grouped because of evidence of monophyly . . . ."

Tayana, you are right that this list isn't as clear as it should be. Brent Mishler pointed out to me that species need not be monophyletic under that position, but I think it is an implication of the Monophyletic position that they must be, or else they aren't diagnosable. Basically there needs to be some "reciprocal monophyly", not that all lineages in the species are monophyletic. I'll check with Wheeler, who I know, about this.

That said, I am presently revising that section for my book. Any suggestions or aid in clarifying and correcting my pre-thesis mistakes of interpretation would be appreciated.

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